1. Field of the Invention
The invention relates to a process for determining the absolute value of a rotational angle, in particular the angular position of a motor vehicle steering wheel, by means of a transceiver comprising a light source and a sensor array having a plurality of optoelectronic transducers, and a code carrier arranged so that the code carrier can rotate with respect to the transceiver, the code carrier has a unique, single-track encoding of the angular values, a continuous segment of the encoding being imaged on the sensor array, and the output signal of the sensor array being evaluated to determine a code word corresponding to the current angular value.
2. Background Art
The absolute angular position of the steering wheel, also called the steering angle, is needed in motor vehicles so that this value can be supplied to a vehicle movement dynamics control system, for example. In addition to the mentioned steering angle value, such a vehicle movement dynamics control system receives other measurement data, such as the wheel speed or the rotation of the motor vehicle about its vertical axis. First the absolute steering angle and second the steering speed are needed so that these values, along with the other captured data, can be evaluated by the vehicle movement dynamics control system and converted to control actuators, for example the brakes and/or the engine management system.
DE 40 22 837 A1 discloses an optoelectronic steering angle sensor that is suitable to accomplish such a process. The steering angle sensor described in this document comprises an electronic control unit and a sensor unit consisting of two elements that are arranged so that they are parallel to one another and at a distance from one another—a light source and a line sensor—and, arranged between the light source and the line sensor, an encoder disk, which is connected with the steering shaft in a torsionally rigid manner. The line sensor provided is a CCD sensor. The encoding provided in this encoder disk is a light slit extending over 360° in the shape of a spiral of Archimedes. The illumination of corresponding transducers of the line sensor at a certain steering angle makes it possible to derive information about the real steering angle position. The spiral of Archimedes used as the encoding is continuous, so that it can be spoken of as an analog encoding. However, using the same arrangement it is just as possible to read a digital encoding on the encoder disk.
DE 197 58 104 A1 (corresponding to U.S. Pat. No. 6,459,389) discloses a process for determining the absolute value of a rotational angle. This process involves evaluating sharp signal changes, generally referred to as edges, in the sensor array's output signal to convert the signal into a contrast difference. A microcontroller decodes the contrast differences by comparing them with a black-and-white pattern representing the angle information.
Although in theory this process is able to achieve the goal of determining the sought-after angular value, it is very sensitive to any kind of interference. For example, dirt in the optical system can cast additional shadows, which also manifest themselves as edges in the electrical output signal, and thus cause additional contrast differences not corresponding to the encoding. Especially when used in an automobile, electromagnetic interference also occurs, which causes additional edges in the sensor array's output signal. However, since the evaluation process is based precisely on identifying and evaluating such edges, or rather the contrast differences derived from them, such spurious pulses cause corresponding measurement errors. Although such measurement errors can be recognized by means of appropriate procedures for plausibility checking of the measurement results derived from them, correction of these measurement results is impossible.